The demand for fire retardant rigid polyurethane foams has increased sharply in recent years as have the governmental and industry requirements and standards that these foams must meet before they may be used in the many areas where their application is particularly desired; notably, the construction industry.
Methods for imparting fire retardancy to polyether and polyester urethane foams heretofore have included the physical incorporation of unreactive fillers and additives; the addition of flame-retardant compounds containing functional groups which become chemically bound in the polymeric urethane chain; and the coating of the inflammable foam with flame-retarding materials.
Among the unreactive additives have been inorganic and organic substances. Illustrative of the inorganic additives have been metal oxides such as Sb.sub.2 O.sub.3, ZnO and Al.sub.2 O.sub.3. It has also been suggested heretofore to incorporate a combination of these unreactive additives to secure good fire retardancy.
The use of hydrated alumina and antimony oxide as additives in low density, flexible and semi-flexible polyurethanes incorporating halogen-containing polymers, such as polyvinyl chloride, has also been suggested, illustratively in U.S. Pat. No. 3,810,851. There is also described in U.S. Pat. No. 3,737,400 a polyurethane foam said to possess self-extinguishing characteristics wherein the flame-suppressing agent is ammonium sulfate together with aluminum hydroxide hydrate and KC1, K.sub.2 O, KNO.sub.3, Ca(OH).sub.2, Mg(OH).sub.2, K.sub.2 SO.sub.4 and Ba(OH).sub.2.
Further illustrative of the additives for use in rendering polyurethane foams flame retardant and reported heretofore in U.S. Pat. No. 3,262,894 is tris-(2-chloroethyl) phosphate in combination with alumina trihydrate.
The improvement in fire retandancy provided by the foregoing additives has often been obtained however at some sacrifice in physical properties. Thus, load-bearing capacity and closed cell content have been found to decrease frequently while moisture vapor pressure transmission often increases. The strength properties of humid aging at elevated temperatures are usually reduced considerably, as well.
To retain the inherent physical properties of the foam considered desirable for use in construction, appliances and the like while imparting an adequate fire retardancy thereto has thus involved a compromise between desired objectives.
One means for alleviating this compromise has been by incorporation of a flame-retarding moiety in the polymer chain itself. Thus, polyurethane foams prepared from the reaction of a polymeric isocyanate; an organic compound incorporating active hydrogen-containing groups reactive with isocyanate moieties; and a dibromobutenediol have, for example, been suggested heretofore. These polyurethanes have been characterized by a significant fire retardancy and have been described broadly for use in fibers, foams, and particularly flexible foams. films and the like. It has also been known to prepare rigid foams manifesting a thermal and oxidative stability considered adequate in various applications utilizing thermal insulation by inclusion in the cross-linked urethane polymer of conventional isocyanurates resulting from isocyanate trimerization in the production of the rigid foam.
While it has thus been known heretofore to prepare various cellular polyurethane foams imbued with a certain desirable thermal stability including rigid foams appropriate for use as rigid thermal insulation in construction materials and appliances, the best of these foams do not have a thermal stability adequate to meet all of the rigorous standards laid down by government and industry for use of these foams in particular applications; at least without the inclusion of additional thermal barriers requiring a special laminate construction made, for example, of perlite and vermiculite.
Accordingly, if the flammability, in terms of flame retardancy and anti-scorching properties of rigid polyurethane foams for use in construction materials and the like could be substantially improved without adversely affecting the properties of these rigid foams which render them otherwise particularly useful, and permits this objective to be accomplished without a material increase in the cost of production thereof, a significant advance in the state of the art would be achieved.